Waterjetting Technology - Dr. Andrej Lichtarowicz

I was saddened, this week, to hear of the passing of Dr. Andrej Lichtarowicz, who died on the the 6th of this month. As Mark Fairhurst noted, Dr. Lichtarowicz, on the faculty at Nottingham University, was a sterling early contributor to the waterjetting community, from its birth back in the early 1970’s. He gave a paper at the first BHRA conference in 1972, and serving as the editor of the 11th Proceedings in 1992.

There were 201 delegates to that first meeting, which was held at the University of Warwick, in Coventry, UK. with 37 papers being given over the course of two and a half days. The use of high pressure waterjet technology was very new at the time, and this was the first time I was able to get together with peer scientists from around the world to discuss what they were doing as well as make a small contribution of our own. But the papers that influenced our lab the most were the two given on cavitation. (One was given by Dr. Andrew Conn and the other by Dr. Lichtarowicz). The reason for this interest was that back in those days the pressures available from high-pressure pumps were restricted to about 30,000 psi, and (without abrasives which only showed up about eight years later) this significantly limited what materials could be cut.

Dr. Conn’s paper related to cavitation at lower pressures and higher volume flow rates, while Dr. Lichtarowicz’ paper covered smaller jet testing at pressures of up to 10,000 psi. Using such a jet he had been able to drill holes in aluminum, which he could not do when the jet was not cavitating. The results were sufficient that we shortly thereafter tried to repeat , and were able to exceed, these results, drilling a hole in a small piece of alumina in less that a minute, although at a pressure of around 18,000 psi.

This led to considerable discussion at the 2nd conference, which was held in Cambridge in 1974, as to whether the results that were being reported were because the jet was breaking up into droplets, or if the result was true cavitation. It was a discussion that Dr. Lichtarowicz, as always, took a significant part in, and although he could not make the third conference (which was in Chicago) by the time of the fourth, in Canterbury in 1978, he was carrying out his research with the nozzle and target submerged with enhanced results from the earlier work.

Over time he developed a small cell, with windows so that the action of the jet could be seen.

Figure 1. Initial design of the Lichtarowicz Cell

The small size of the unit, and the relative simplicity of construction, meant that a number of us, around the world, built such units and used them to help develop a better understanding of what was happening, and how damage could be increased.

One of the early discoveries he made was that, by adjusting the pressure in the chamber, the amount of overall damage (measured by mass loss) could be significantly intensified, and the rate of erosion increased. It was on that basis that we, among others, were able to use cavitating jets to disaggregate rock and coal into fine particles.

Figure 2. View through the port of a Lichtarowicz cell, showing the cavitating jet impacting a metal target.

One of the major uses of the cell was, however, not as a tool to develop faster ways of drilling rock (though it did) but instead to accelerate the rate at which the cavitation resistance of different materials could be determined. Until that time the standard tool for determining cavitation resistance had been the vibrating horn device recommended by ASTM. The problem with this was that it took hours (typically about 24) to generate the data and plot the rate of material removal, because it was so slow. With the cell a similar result could be obtained in minutes. His work led to the development of an ASTM standard first adopted in 1995, and reapproved in 2001 and 2006, with current interest in revision. It went on to be incorporated as part of the International Cavitation Erosion Test.

And so the technology moved forward, Dr. Lichtarowicz gave his last BHR paper at the 12th Conference in Rouen in 1994, and this was a review of some of his earlier work, showing its relevance as industry sought to find cleaner, greener methods for cleaning and material removal. His fundamental work, however, fostered studies that continue to live on, particularly in Japan, where laboratories continue to develop the techniques and ideas that he pioneered over the years. Certainly our own work would not have progressed as far, or in as many directions, without the inspiration of his work, and the many discussions on the technology we held over the years.

He was a good friend, not only to young faculty – as I was when we first met – but to the industry as a whole, and the students that he taught over the years. He was a much respected scientist and colleague and the tools that he developed and helped us learn to use will continue.

Tech Talk - Energy cost, additive engineering and cavitation

I paid $2.85 for a gallon of gasoline this weekend, at the gas station just up the road from our house, here in South Central Missouri. A couple of weeks ago while I was in the UK the price my brother paid was around $8.00 a gallon. The BBC calculator that I used to check the UK price tells me that I am paying $6.89 less per tank than the regional average here, and that were I to live in Italy my tank-full would have cost me $95 more, while it Venezuela it would have cost $43 less. (It cost $45 to fill my tank).

The low cost of fuel is one of the benefits from the increased crude oil production in North America, sustained as it is by the increase in production from Saudi Arabia to balance the global market losses from other countries around the world. Further the EIA explains the refineries are helped with this low price by the high demand for diesel and the premium that it has achieved – causing refineries to run at record levels to meet the demand, and producing, as a secondary product, more gasoline that is thus being marketed at the lower price. It is a situation that the EIA expects to continue for a while.

Figure 1. US refinery inputs (EIA TWIP Nov 6, 2013)

The relatively low price of fuel, here in the United States, particularly relative to Europe is starting to attract industries historically located abroad. The move to date is being led by those attracted by the cheap price of natural gas, particularly in the chemical industry. BASF, for example, cut the ribbon last week on a plant expansion in Vidalia, LA and just recently announced plans to expand its research facility in Beachwood, Ohio.

It was, however, another report on manufacturing that really caught my attention this week. It was the news that 3D Printer technology had advanced enough to now make a gun from metal parts. The process involved is somewhat more complicated than that used in earlier guns manufactured using this new generation of equipment. Earlier in the year a gun had been made from plastic parts and made some additional news when a version fired nine shots without falling apart. The evolution of the plastic gun is worth noting in that the first one reported was built from components printed with an $8,000 second-hand Stratasys Dimension SST 3D printer. And while it fired a shot successfully, the gun blew up on the second trial. The second gun, however, was made on a $1,725 Lulzbot A0-101 3D printer, that was available from Amazon, made by Aleph Objects and it survived firing nine rounds. For a variety of reasons the plastic gun contained some metal parts, but it marked the advent of this new technology. Prices for these replicator units are already down below $2,000 and they are limited, at present, to working with different types of thermoplastic. (But they can make, for example, shoes.)

The difference in being able to move to making parts from metal, particularly those that allow the repeated (over 600 times) firing of the gun is a very significant step forward. Thirty-four parts were made from stainless steel and Inconel 625 and then a grip was made from nylon, using a classic 1911 design.

Figure 2. The metal gun made by Solid Concepts (Solid Concepts )

It is the different metal part of this that is worth underlining. The components were made by laser-sintering (which simplistically means that they used a laser to melt tiny particles of metal so that they would fuse together to make the model). The machine that is used to do this, at the present time costs between $400,000 and $1,000,000. It also has power and other logistic needs that require it be run in a commercial, rather than residential environment.

But, as Sold Concepts notes:

Solid Concepts has been using metal sintering for some time now to successfully create parts for a wide array of products. The 1911 gun is well known and people can relate to it in respect to its power and need for precise components. This story is about how additive manufacturing can be used to produce real, accurate parts in your industry whether it’s aerospace, transportation, medical, energy, consumer products, etc.

The changes that this will make in industrial manufacturing, and in the global market for materials cannot be underestimated. At present parts are generally made by subtraction, taking large billets of material and milling and machining away all the un-needed bits, producing large volumes of scrap chips. None of that waste will be generated with this new process.

Chris Hechtl has already produced The Wandering Engineer” series of Science Fiction books, starting with New Dawn that uses the concept widely as one of the bases for the stories. (Worth a read just to get some idea of the scope of what is to come - though I am also enjoying the series, as the books are written).

It is going to change the way in which components are built, but it will also change the way in which minerals are processed once they are mined from the earth. It will be no longer necessary to cast metals into large ingots and then forge them down into smaller shapes. It is likely that, for many items in the near future that process will still be cheaper, but as time progresses and the costs of the process reduce (bear in mind that this is laser-based and remember how those costs have come down as lasers have become ubiquitous in society) that even large parts may be better made this way. Further it allows intricate melding of different materials to make products that are stronger and better suited to the need.

Thus the objective of mineral processing in the years to come will be aimed at making fine powders rather than going through all the steps to make the larger ingots. That will, in turn, impact earlier stages of processing, and, while I don’t normally discuss my own work in these posts, I would draw your attention to a recent post from October 31st, down below, which includes a video of a small piece of equipment virtually instantly breaking half-inch coal into 5-micron pieces, which can be done with a pressure washer from the local hardware store. It also works in breaking out minerals from their host rock.

The world indeed will change, and with those changes the power requirements of the future are also going to undergo drastic revision.

Waterjetting 12d - The heat of a waterjet cut

In the last three posts I have been discussing the quantity of heat that is created when machine tools are used in the cutting of rock, metals and other materials. The amount that the temperature of both the cutting tool and work piece material will increase, and the effect that this has on the cutting tool and the finished part can, as I have shown, be reduced if a quite small stream of high-pressure water is directed into the small zone where the cutting is taking place.

But what happens if the cutting process doesn’t use the large scale typical mechanical cutting tools, but instead uses the very small particles embedded within the jet stream itself as part of an abrasive waterjet cutting system? For many years the evidence, after the cut was over, indicated that there was very little heat build-up in the part, and the process appeared to be a “cold cut,” but there was no immediate evidence, because of the rapidity with which the cut was made. However, with advances in technology that limitation was removed, and research scientists at the University of Hannover have now been able to make temperature measurements during cutting. (A Thermographical Map of Tool and Workpiece During the Cutting Process by Plain Waterjet and Abrasive Waterjet up to 900 MPa, H. Louis, A. Schenk, F. Pude and M. Mohamed, 17th International Conference on Waterjet Cutting Technology).

The group used an infra-red camera connected into a computer to capture images as an abrasive waterjet cut into a target work sample. The instrument had been calibrated to show the color temperatures that the image revealed.

Figure 1. Temperatures read through an infrared camera as an abrasive jet cuts into a target plate. (H. Louis et al, ibid)

The arrangement by which the images were obtained was relatively simple:

Figure 2. Experimental arrangement allowing capture of the temperature build-up in the cutting head, the abrasive jet and the work piece during an AWJ cut (H. Louis et al, ibid).

During the course of the experiment the size of the cutting jet and the pressure were changed to find how these controlled the temperatures that were generated in the different parts of the operation. The work first examined the results when only a plain waterjet, without abrasive particles, was used in cutting.

Figure 3. Temperature build-up when plain waterjets (at 125,000 psi) are being used to cut a piece. (H. Louis et al, ibid)

Note that there is not a large amount of heat generated in the part, in this case a temperature rise to 126 Deg F was measured, though the temperature rise in the nozzle holder was similar in range. When the effects of jet flow and pressure were plotted, the role that an increase in pressure played in raising the part temperature around the cutting zone is clear. Note, in Figure 3, the region over which the temperature has been raised in the work piece.

Figure 4. Temperature rise in the nozzle holder as a function of jet pressure. (H. Louis et al, ibid)

Note that at pressures of up to 100,000 psi (700 MPa) the temperature rise is only up to 86 deg F, much less than that in conventional mechanical cutting.

When abrasive is added to the jet stream, then the temperatures generated, as Figure 1 indicated, are higher in the nozzle holder, because of the impact of the particles with the focusing tube as part of the particle acceleration. The piece was moved under the jet at 1.2 inches/minute, with an abrasive feed of 0.06 lb/minute, with jet pressures varied from 42,000 psi to 115,000 psi. (300 to 800 MPa). The target was a metal alloy.

Not surprisingly as the pressure in the jet increased, so did the temperature in the focusing tube.

Figure 5. Temperature increase in the focusing tube, as a function of jet pressure (H. Louis et al, ibid).

Temperatures were measured at the top, middle and bottom of the cut which the AWJ made through the target material, and these are shown in the following plot:

Figure 6. Temperature build-up in the work piece during the cutting operation (H. Louis et al, ibid).

The graph shows that the temperature build-up is greatest in the middle of the cut, although this difference is small, and begins to disappear as the jet pressure increases. At 100,000 psi the temperature can rise to 150 deg F.

In most cutting work that temperature rise would not be enough to cause any damage to the part being cut. Where very temperature sensitive materials have been cut with the jet at lower pressures and higher speeds at MS&T the zone of influence of the cutting operation was measured in microns.

It is in living tissue, which can be more sensitive to temperature, where this can be a problem. The University in Hannover is internationally recognized for the work that it has been carrying out in to the use of high pressure waterjets in medical applications. While this is a subject for another day (or several since the range of applications continues to grow from year to year) the caution comes from work on cutting bone and reported at the 18th International Jet Cutting Conference in Gdansk by Biskup et al “Temperature measurement during abrasive water jet cutting of cortical bone measured by thermocouples”). Bear in mind, however, that one of the problems that the technology is seeking to address in these bone cutting experiments is to achieve a better quality cut than can be achieved with a hand saw, which has often been the tool used by a surgeon when dissecting bone, and the required edge quality is sometimes more difficult to achieve with that tool.

Figure 7. Temperature build-up in bone under varying conditions and for two bone thicknesses, as a function of residence time. (Biskup et al, ibid)

It can be seen that a thicker bone sample does become vulnerable to too high a temperature if there is a significant exposure time before the part is pierced. However, with an appropriate selection of parameters the temperature can be kept down in a range where the tissue does not die, and the considerable advantages to jet use can therefore be used.

Keeping the parts being cut cool is important in very delicate and precise work, where thermal distortion of the metal, particularly in thin but deep cuts, can otherwise lead to unacceptable failures to maintain tolerance.

Waterjetting 12c - Jet assisted metal cutting

The first two posts in this section described how, in cutting through rock, the tool and the rock would be compressed together so that temperatures could be created in and around the tool that would exceed 2,000 deg C. That temperature is sufficient to melt the cutting tool, and in other situations is hot enough that it can ignite pockets of gas in underground operations that can have fatal results. However, by adding a small flow (less than 1 gpm) of water to the cutting pick not only is this risk of gas ignition or pick melting significantly diminished, but the water acts to remove the fragments of the rock as they are broken under the bit. This has two beneficial effects, first it removes the small rock that would otherwise be re-crushed and rub against the bit, causing the temperature rise due to friction. The second is that by also keeping the tool cool and sharp it can penetrate much deeper into the rock under the same forces, improving the efficiency of the cutting.

When a cutting tool is used to cut metal instead, the processes are somewhat different. However, because the tool rubs against the metal and cuts and deforms the metal that will be removed as a chip heat will still build up around the cutting zone.

Figure 1. Temperatures around a cutting tool in metal (Gear Solutions Magazine )

If you look closely at the temperature contours you will see that the lines stretch beyond the point where the cut is being made, and both the chip and the machined surface of the metal heat up to 500 degC. This narrow strip of metal on the surface of the piece is referred to as the Heat Affected Zone or HAZ, since the metal in this region has had its properties changed by the heat and deformation. And while the impact is more severe with a thermal method of cutting (such as plasma) there is some affect with mechanical cutting.

This can be seen, for example, if a metal piece is machined without cooling of the interface between the bit and the chip. Depending on the material being cut, this can lead to chips that are thermally damaged, are long and can be dangerously hot.

Figure 2. Strips of metal milled without cooling (Dr. Galecki)

If the surface of the chips are examined then the amount of heat damage is evident.

Figure 3. Surface of the chip showing the damage from the heat during cutting. (Dr. Galecki)

However this problem with the heat generated during cutting has been widely recognized, and so it has become standard practice to play a cooling fluid over the cutting zone during machining. To be effective the water must pass into the passage along the tool face and down into the cutting zone. It thus acts both to lubricate the passage of the chip up the blade, and separating it from the cutting tool, while cooling the bit and keeping it sharp.

Figure 4. Insertion of the jet into the cutting zone. (Dr. Mazurkiewicz)

When this is properly placed, and as with the jet assisted cutting of rock the precision required in placing the jet is around 1.10th of an inch, then the chip and metal surface are cooled and the tool remains sharp.

However, with conventional, lower pressure cooling, while the chip length is reduced and the surface is somewhat improved, overall cutting forces do not change.

Figure 5. Chips formed with conventional cooling (note the poor edge quality). (Dr. Galecki)

When the waterjet pressure is increased to the ultra-high pressure range, so as to ensure that adequate water reaches the tool, then the cutting forces are reduced and the amount of damage to the metal is further reduced

The result can be seen in the form of the chips that are removed, which are now much shinier in appearance:

Figure 6. Chips from high-pressure jet assisted cutting (Dr. Galecki)

Note that the surface of the chips are shiny, and that they are relatively small in size. The shiny surface is similarly reflected in that left on the machined part.

Figure 7. Cut surface left after high-pressure jet assistance to the cutting tool.

The resulting reduction in damage to the machined surface, as well as the lower machine forces, and the consequent lowering of the potential for “chatter” during cutting gives a higher cut surface quality which, because of the reduced damage to the surface has a higher fatigue resistance.

The amount of modification required to the equipment is not necessarily large, since the high pressure water can be carried to the tool through relative small tubing that has a small footprint. The pump can be located elsewhere. Further, while conventional cooling requires additives to the water (which make it more costly to treat the scrap) the clean water used in the jet makes this less of a concern.

Figure 8. Arrangement with a jet added to the cutting tool on a lathe. There are also instruments on the platform. (Dr. Galecki)

These results show that the heat damage that can be anticipated with conventional machining of metal can be significantly reduced with the addition of high-pressure water. This becomes even more clear where abrasive is added to the jet stream, and fortunately, thanks to colleagues in Germany, we have thermal images of this, which I will share, next time.

(For further reading see Mazurkiewicz, M., Kabala, Z., And Chow, J., "Metal Machining With High Pressure Water Cooling Assistance - A New Possibility," ASME Journal of Engineering for Industry, Vol. 111, February, 1989.)